Tightly packed, organic-free arrays of nanoparticles are useful for optical, magnetic, and electronic device applications. There are existing approaches for forming packed nanoparticles, employing organic ligands in solution to form assemblies. The presence of organic ligands is problematic because the ligands can melt from heat, darken in the presence of light, and inhibit electrical conductivity.
Nanoparticles may be crystallized from solution. Arrays of nanoparticles from crystallization methods are usually surrounded by organic ligands that interfere with further chemical processing and can limit durability. Arrays of nanoparticles from crystallization methods are not protected by an outer layer, which makes them susceptible to environmental damage. Other organic molecules such as surfactants, polymers, or DNA may be used to induce crystallization or assembly.
Nanoparticles may be formed in an array on a substrate. The nanoparticles may be packed using drying from a solution (colloidal crystals) or electrophoresis, both requiring a substrate. Colloidal crystals are formed by dispersing colloids in a solution and drying on a substrate or spin coating on a substrate. Electrophoresis does not necessarily result in tightly packed nanoparticles. Arrays of nanoparticles may be grown in an array on a substrate from a process with gaseous reactants. Nanoparticles may be assembled into aligned arrays under a magnetic field.
Arrays of nanoparticles or nanorods may be formed through a wet chemical process. In one approach, metals or oxides are deposited in the pores of anodic alumina or mesoporous silica. This approach forms either single layers of nanorods or nanorods that have empty space between them and no material between the rods. This results in a low density of material and reduced efficacy from the array.
The packed arrays of nanoparticles, as taught in the prior art, tend to be non-uniform in array size and shape. In order to be useful in larger devices, arrays of assembled particles should be uniform in size and shape. This is necessary for either bottom-up assembly processes, such as self-assembly into larger assemblies; for top-down assembly processes, such as pick-and-place assembly onto patterned substrates; or for combinations of bottom-up and top-down assembly.
What is desired is a method of forming multiple layers of packed nanoparticles, without relying on organic ligands and without relying on a substrate. Devices configured to carry out the method are also needed.